A mass flow controller is widely used for the purpose of controlling a flow rate of gas supplied to semiconductor manufacturing equipment, or starting or stopping supply of gas etc., for example. The mass flow controller comprises a flow control valve, a flow meter and a control means.
A valve opening of the flow control valve is changed (increased and decreased) according to a control signal which the control means outputs. Here, the “valve opening” means a value corresponding to a cross-sectional area of the narrowest part of a path through which gas passes inside the flow control valve. The valve opening of the flow control valve is controlled so as to become an arbitrary value between the minimum value (for example, zero) corresponding to the minimum value of the above-mentioned cross-sectional area and the maximum value corresponding to the maximum value of the above-mentioned cross-sectional area. When the valve opening is the minimum value (for example, zero), gas does not flow. When the valve opening is maximum, a flow rate of gas, which the mass flow controller supplies, becomes maximum.
The flow meter measures a flow rate of gas which passes through the flow control valve. Generally a calibrating (calibration) of the flow meter is performed using reference gas (for example, nitrogen gas) at a certain reference temperature (for example, 22° C.). When controlling a flow rate of gas of a different kind from the reference gas, an error occurs in the flow rate of the gas measured by the flow meter since physical properties (for example, specific heat, etc.) of the gas are different from physical properties of the reference gas. In this case, the error can be corrected using the conversion factor (CF) acquired beforehand for each kind of gas (see for example, Japanese Patent Application Laid-Open (kokai) No. H08-54268). Moreover, when temperature of the gas is different from that of the reference temperature, the flow rate of the gas measured by the flow meter can be corrected based on the temperature of the gas (see for example, Japanese Patent Application Laid-Open (kokai) No. 2004-93174).
The control means outputs a control signal to the flow control valve to control the valve opening such that a flow rate of gas measured by the flow meter matches a set flow rate. In case of what is called a normally closed type flow control valve, the valve opening becomes minimum (zero) and the flow rate of the gas also becomes zero when intensity of the control signal (voltage intensity or electric-current intensity) is minimum (zero), while the valve opening is maximum and the flow rate of the gas also becomes maximum when the intensity of the control signal is maximum. On the other hand, in case of what is called a normally open type flow control valve, the valve opening is maximum and the flow rate of the gas also becomes maximum when the intensity of the control signal (voltage intensity or electric-current intensity) is minimum, while the valve opening becomes minimum (for example, zero) and the flow rate of the gas also becomes minimum (for example, zero) when intensity of the control signal is maximum. Thus, the valve opening of the flow control valve is changed depending on the intensity of the control signal, and the flow rate of gas passing through the flow control valve also changes. The control means can perform feedback control, in which the flow rate of gas measured by the flow meter is used as a controlled variable, as a method for controlling the flow rate of gas.
When a valve opening of a normally closed type flow control valve is changed by using a piezoelectric element, for example, the valve opening is proportional to voltage intensity impressed to the piezoelectric element. For example, in a case where the voltage intensity impressed to the piezoelectric element is a maximum value (50 V) and the flow rate of gas at that time is 1 slm (standard litter per minute) when the valve opening is maximum, the valve opening will become 50% of the maximum value and the flow rate of the gas will become 0.5 slm when the voltage intensity impressed to the piezoelectric element is set to 25 V (50% of the maximum value). On the other hand, when a valve opening of a normally open type flow control valve is changed by using a piezoelectric element, the valve opening becomes maximum when the voltage intensity impressed to the piezoelectric element is zero, and the valve opening becomes smaller as the voltage intensity impressed to the piezoelectric element increases. For example, in a case where the minimum voltage intensity impressed to the piezoelectric element for making the valve opening to zero is 50 V, the valve opening will become 50% of the maximum value and the flow rate of the gas will become 0.5 slm when the voltage intensity impressed to the piezoelectric element is set to 25 V. In addition, the correspondence relation between the voltage intensity impressed to the piezoelectric element (intensity of the control signal) and the valve opening, which was shown in the above-mentioned explanation, is only an example, and the scope of the present invention is not limited by those specific numerical values.